A flat strain wave gearing comprises first and second internally toothed gears arranged in parallel, a cylindrical externally toothed gear arranged therein, and a wave generator arranged inside the externally toothed gear. Typically, the first internally toothed gear is a stationary-side internally toothed gear fixed so as not to rotate, and has more teeth than does the second internally toothed gear. The second internally toothed gear has the same number of teeth as does the externally toothed gear, and is a drive-side internally toothed gear capable of rotating integrally with the externally toothed gear. The externally toothed gear is flexed into a non-circular shape by the wave generator, the externally toothed gear partially meshing with the first and second rigid internally toothed gears.
When the wave generator is caused by a motor or the like to rotate at high speed, the positions at which the externally toothed gear meshes with the first and second internally toothed gears move in the circumferential direction. Relative rotation is generated between the first internally toothed gear and the externally toothed gear according to the difference in the number of teeth. Because the first internally toothed gear is fixed so as not to rotate, the externally toothed gear rotates. The rotation of the externally toothed gear is delivered from the second internally toothed gear rotating integrally with the externally toothed gear, and is transmitted to a load-side member.
Patent Documents 1 and 2 disclose flat strain wave gearings in which ball bearings comprising flexible inner and outer races are used as wave generators. Patent Document 3 discloses a flat strain wave gearing in which a needle roller bearing comprising flexible inner and outer races is used as a wave generator. Patent Document 4 discloses a flat strain wave gearing comprising a wave generator that comprises a pair of rollers.